VAD integrated flow sensor

ABSTRACT

A blood pump with an integrated flow sensor is provided. The blood pump may include an implantable pump for pumping blood having a housing, a flow path extending within the housing and at least one movable element within the housing for impelling blood along the flow path and a sensor for measuring the flow rate of blood through the pump. According to one embodiment, the sensor may be mounted to the housing of the pump. In accordance with a further embodiment, the housing may have an exterior surface defining a cavity, and the sensor may be located within the cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the filing date of U.S.Provisional Patent Application No. 61/697,087, filed Sep. 5, 2012, thedisclosure of which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to blood pumps usable as implantableventricular assist devices, and more particularly to an improved bloodpump device with an integrated ultrasonic flow sensor.

In certain disease states, the heart lacks sufficient pumping capacityto maintain adequate blood flow to the body's organs and tissues. Forexample, conditions such as ischaemic heart disease and hypertension mayleave the heart unable to fill and pump efficiently. This condition,also called congestive heart failure, may lead to serious healthcomplications, including respiratory distress, cardiac asthma, and evendeath. In fact, congestive heart failure is one of the major causes ofdeath in the Western world.

This inadequacy of the heart can be alleviated by providing a mechanicalpump also referred to as a ventricular assist device (“VAD”) tosupplement the pumping action of the heart. VADs may be used to assistthe right ventricle, the left ventricle, or both. For example, a VAD mayassist the left ventricle by mechanically pumping oxygenated blood fromthe left ventricle into the aorta. In this case, the pump is implantedwithin the body of the patient, an inflow conduit is attached to theleft ventricle, and an outflow conduit is attached to the aorta. Forexample, where the pump is implanted below the heart or at the bottom ofthe heart, the outflow conduit may be a flexible conduit extendinggenerally upwardly, from the outlet of the pump to the aorta. The pumpreceives blood from the left ventricle and then pushes it into the aortafor distribution throughout the body. This reduces the strain on theheart by reducing the volume of blood that the heart is responsible formoving.

U.S. Pat. Nos. 7,575,423, 7,976,271, 8,007,254, and 8,419,609, thedisclosures of which are hereby incorporated by reference, disclosecertain rotary blood pumps which can be used as ventricular assistdevices. These pumps are electrically powered. Typically, these andother electrically powered implantable pumps are connected through acable, commonly referred to as a “driveline”, to a control device whichsupplies electric power to the pump and controls its operation. Thecontrol device may be external to the patient's body, in which case thedriveline extends through the skin. It has also been proposed to useimplanted control devices which receive power from an external source bymeans of an implanted induction coil.

It is desirable to monitor certain parameters of the pump, including forinstance the rate of blood flow through the VAD. Flow information can beused to detect abnormal operating conditions, such as blockage of theoutflow conduit or a “suction” condition, where the left ventricle isnot refilled fast enough to keep the pump supplied with blood, and alsocan be used to provide feedback control of the pump. However, blood flowthrough a VAD is difficult to monitor because it often cannot bemeasured directly. It would not be desirable to install a bulky sensorin the path of the flowing blood, as the sensor could obstruct the bloodflow and reduce the effectiveness of the pump.

One solution that has been proposed is to measure blood flow indirectly.This can be achieved by measuring blood pressure at both the inflow andoutflow sections of the pump, and then mathematically computing bloodflow. Pressure sensors have been incorporated into VADs for the purposeof monitoring blood flow through the VAD. Blood flow also can bedetermined indirectly from operational parameters of the pump as, forexample, the speed of the pump and the power used by the pump.

Other solutions have been proposed that involve measuring blood flowthrough the pump directly. This can be achieved, for instance, throughthe use of an ultrasonic flow probe. For example, it has been proposedto provide an ultrasonic flow probe around mounted on the outflowcannula. Similarly, European Patent EP1046403 discloses a bloodcirculation device with ultrasonic flow sensors attached to the inflowcannula or “blood feeding pipe.” In these proposed solutions, blood flowcan be monitored directly for enhanced control over the therapeuticqualities of the pump. However, these solutions require an additionalstructure to hold the ultrasonic flow probe. Moreover, as furtherdiscussed below, certain types of ultrasonic flow measurement can beused only in a rigid conduit. Where the flow is measured along aflexible conduit, the additional structure typically must haveappreciable bulk to hold a portion of the flexible conduit in a fixedconfiguration. Also, these arrangements require an additional cableextending to the additional structure housing the flow probe. Thesefactors make it more difficult to implant the system in the body.

Thus, despite very considerable effort devoted in the art to developmentof ventricular assist devices, further improvement would be desirable.Particularly, there is a need for a VAD which can provide the benefitsof direct flow measurement without substantially increasing thedifficulty of implanting the device.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a blood pump with anintegrated flow sensor. The blood pump according to this aspect of theinvention desirably includes an implantable pump for pumping bloodhaving a rigid housing, a flow path extending within the housing and atleast one movable element within the housing for impelling blood alongthe flow path, and a sensor for measuring the flow rate of blood throughthe pump. In this aspect of the invention, the sensor may be mounted tothe housing of the pump.

A further aspect of the invention provides a blood pump including afirst housing element having an interior surface at least partiallydefining the flow path and having an exterior surface defining a cavity.In this aspect, the sensor may be located within the cavity. The sensormay also include, for example, one, two or more ultrasonic transducers.

Yet another aspect of the invention includes a blood pump having a firstand second platform. According to this aspect, the flow path may extendalong a flow path axis extending in upstream and downstream directions.The first housing element may define a first platform facing downstreamat an oblique angle to the flow path axis and a second platform surfaceupstream at an oblique angle to the now path axis. Further, theultrasonic transducers may include a first transducer mounted to thefirst platform and a second transducer mounted to the second platform.The platforms may, for example, have a slope of substantially 45 degreesto the flow path axis. The ultrasonic transducers may also be mounted tothe platforms with an adhesive.

A further aspect of the invention also provides a second housingelement. In this aspect, the first and second housing elements maycooperatively define at least a portion of the flow path. Further, thetransducers may be arranged such that the ultrasound emitted from one ofthe transducers passes through the flow path to the second housingelement and reflects from the second housing element and passes to theother one of the transducers. The housing may further include a coveroverlying the cavity in the first housing element. Optionally, anelectronic circuit may be disposed within the cavity and may also beconnected to the sensor.

A still further aspect of the invention includes a blood pump having aninflow end and an outflow end. The sensor may be mounted adjacent theoutflow end of the flow path. The sensor may also be mounted adjacent tothe inflow end of the flow path. In some aspects of the invention, thepump may be a rotary pump.

A further aspect of the invention may provide a blood pump having one ormore electrical elements for moving the movable element. In this aspect,the device may also include an external control unit that powers theelectrical elements. Further, the device may also include a drivelinefor connecting the pump and the one or more ultrasonic transducers tothe external control unit. The sensor may be connected to the controlunit through the driveline. The driveline may also be the onlyconnection between the pump and the control unit.

These and other aspects of the invention will be more readily understoodwith reference to the detailed description taken below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a blood pump usable as a ventricularassist device in accordance with one embodiment of the invention, inconjunction with the heart and certain blood vessels of a human patient.

FIG. 2 is an exploded view of pump used in the ventricular assist deviceof FIG. 1.

FIG. 3 is a fragmentary perspective view depicting a portion of theventricular assist device of FIGS. 1 and 2.

FIG. 4 is a detailed view of the ventricular assist device shown inFIGS. 1-3, on a further enlarged scale.

FIG. 5 is a partial exploded view of the ventricular assist device ofFIGS. 1-4.

FIG. 6 is a partial assembled view of the ventricular assist deviceshown in FIGS. 1-5.

FIG. 7 is a diagrammatic cross-sectional view of the ventricular assistdevice shown in FIGS. 1-6 accordance with one embodiment of theinvention.

DETAILED DESCRIPTION

FIG. 1 illustrates an implantable blood pump usable as a ventricularassist device (“VAD”) 100 in accordance with one embodiment of theinvention. In this embodiment, the VAD 100 comprises a pump 110 havingan outer housing 111 that includes and a first or lower housing element113 and a second or upper housing element 112. The housing elements areformed from biocompatible rigid materials such as titanium or a hybridtitanium-ceramic. The upper housing 112 may further comprise an inflowend 114 (FIG. 2) defining an inlet opening 107. In the implantedcondition depicted in FIG. 1, the inflow end of the pump is insertedinto heart of a mammalian subject such as a human patient, typicallyinto the left ventricle so that the inflow opening is in communicationwith the interior of the ventricle. The VAD 100 may also include anapical ring 119 for securing the connection between the outer housing111 of the pump and the heart. The sewing ring typically is sutured inplace on the apex of the heart, and may include a clamp to secure theouter housing 111 to the sewing ring and thus secure the pump in placerelative to the heart.

The VAD 100 may also include an outflow conduit 121 extending from theouter housing 111. The outflow conduit 121 may comprise a flexible,biocompatible main tubing 122. The main tubing 122 may also be encasedalong a portion of its length by an anti-kinking conduit 123. Theanti-kinking conduit 123 may be made by plastic interlocking links toprevent kinking. The main tubing 122 may further be surgically attachedto a desired position 124 of the heart or the surrounding area, such asto the ascending aorta as depicted in FIG. 1.

The VAD 100 may further include a cable 130, also referred to herein asa driveline. Driveline 130 typically includes a plurality of electricalconductors 131. The driveline 130 electrically connects components ofthe pump 110 within the outer housing 111 to an external control unit191. Control unit 191 is arranged to supply electrical power to thepump, and to control the operation of the pump. All or part of controlunit 191 may be implanted within the body of the subject, or may beexternal to the subject.

As shown in FIG. 2 upper housing element 112 and lower housing element113 define a flow path therein, which extends from the inlet opening 107to an outflow end 115 a and 115 b cooperatively defined by the housingelements 112 and 113. Permanent magnets (not shown) may be providedwithin one or both of the housing elements 112 and 113, along with a setof electromagnet coils, one of which is schematically represented at109. In one embodiment, a permanent magnet stack may be contained withina center-post 117 on the lower housing element 113. Pump 110 furtherincludes a movable element 116 for impelling the blood along the flowpath. In this embodiment, the movable element 116 is a wide-bladedimpeller, which incorporates permanent magnets (not shown). Thepermanent magnets within impeller 116 cooperate with the permanentmagnets in the housing elements to keep impeller 116 suspended and outof contact with the housing elements during operation. When the coil set109 is energized with alternating current, the magnetic interactionbetween the coil set and the permanent magnets of the impeller spin theimpeller around its axis, so that the impeller will drive blood alongthe flow path.

The upper 112 and lower 113 housing may further define a drivelineinterface 118 a and 118 b for receiving a power connector 127 on thedriveline into the pump housing. In one embodiment, the upper housing112 defines a top portion of the driveline interface 118, and the lowerhousing 113 defines a bottom portion of the driveline interface 118 b.The driveline interface 118 is provided with appropriate terminals (notshown) for making electrical contact with certain conductors ofdriveline 130 at power connector 127. These terminals are electricallyconnected to the coil set.

As shown in FIGS. 3-8, and as further discussed below, the pump isprovided with a flow sensor carried on or within the housing 111. Ineach of FIGS. 3-6, the first or lower housing element 113 is frontfacing and visible. In this embodiment, the exterior surface of lowerhousing element 113 has a PCB cavity 361 and a further cavity 162 whichaccommodates a first platform 163 and a second platform 166. In oneembodiment, cavity 162 is about 0.5 mm in depth. Preferably, the first163 and second 166 platforms have a height less than or equivalent tothe depth of the cavity 166 so that the platforms may be fullyaccommodated within the cavity 162. As best appreciated with referenceto FIG. 7, platforms 163 and 166 overlie a portion 195 of the flow pathcooperatively defined by housing elements 112 and 113. This portion ofthe flow path is adjacent the outflow end 115 a, 115 b depicted in FIG.2. Within this portion 195 of the flow path, the blood flows in adownstream direction indicated by arrows D in FIG. 7, generally along aflow path axis 193. As best seen in FIGS. 4 and 7, the first platform183 defines a first surface 164 facing away from flow path 195 andfacing generally upstream (the direction opposite to arrows D) at anangle oblique to the flow path axis 193, as well as a second surface165. The second platform 166 is disposed downstream from the firstplatform. Second platform 166 defines a first surface 167 facing awayfrom the flow path and facing downstream at an angle oblique to the flowpath axis 193, and also defines a second surface 168. For example, thefirst surface of each platform may be disposed at an angle of 45 degreesto the axis of the flow path.

A window 172 (FIG. 7) forms an interface between the first platform 163and the interior of the flow path, and another window 172 forms aninterface between the second platform and the flow path. Although thewindows are depicted as separate elements from the platforms for clarityof illustration, the windows may be formed integrally with theplatforms. Also, the surfaces of the windows bounding the flow pathdesirably are flush with the surrounding surfaces of housing element113.

The windows and platforms may be formed integrally with first housingelement 113 or may be fixed directly to this housing element. Thematerials of the platforms and windows desirably provide a low-impedancepath for ultrasound between first surfaces 164 and 187 and the interiorof the flow path when the flow path is filled with blood. For example,the materials of the platforms and windows may have acoustic impedancereasonably close to that of blood so as to minimize reflection ofultrasound at the interface with the blood within the flow path. Forexample, the platforms and windows may be formed by casting abiocompatible polymer.

A first ultrasonic transducer 142 is bonded to the first surface 164 ofthe first platform 163, whereas a second ultrasonic transducer 144 isbonded to the first surface 167 of the second platform 166. Theultrasonic transducers may be conventional piezoelectric elements. Thetransducers are electrically connected by conductors 145 and 146 toelectronic components on a printed circuit board 150 (FIGS. 5 and 6)which is disposed in cavity 161 of housing element 113. The componentson the printed circuit board may include conventional components fordriving one of the transducers (referred to herein as the “driventransducer”) at an ultrasonic frequency, typically in the megahertzrange with an electrical signal, and for amplifying electrical signalsfrom the other one of the transducers (referred to herein as the“receiving transducer”). The electronic components may also includecomponents for comparing the phase of the electrical signals from thereceiving transducer with the phase of the signals applied to drive thedriven transducer.

Printed circuit board 150 is connected by conductors 151 to conductors198 of driveline 130 at a connector 197 engaged in an opening 199 (FIG.5) communicating with cavity 561. In this embodiment, connector 197 isseparate from the power connector 127 (FIG. 1) engaged with thedriveline interface 118 (FIG. 2). The conductors associated withconnectors 197 and 127 extend within the outer sheath of driveline 130over most of the length of the driveline, and diverge from one anotheronly in the immediate vicinity of the pump. Conductors 198 of thedriveline link the components on PCB 150 with an appropriate circuit inthe control unit 191 (FIG. 1).

A cover schematically indicated at 173 (FIG. 7) overlies cavities 161and 162, and thus forms a part of the housing which cooperates withhousing element 113 to enclose the platforms, transducers, printedcircuit board and associated conductors. Cover 173 may be abiocompatible potting material such as an epoxy, or may be a plate fixedto housing element 113 by appropriate fasteners and sealed by anappropriate gasket to prevent entry of body fluids into the cavities 161and 162.

In operation, with the pump operating and forcing blood through the flowpath, the control unit actuates the components on PCB 550 to drive oneof the transducers. For example, the control unit and components on PCB550 may cause the first or upstream transducer 143 to emit ultrasonicwaves. These waves pass along a path 174 at an oblique angle to thedirection of the blood flow (the downstream direction) and impinge onthe wall of the flow path defined by the second or upper housing element112 at a point 175. The ultrasonic waves are reflected along a furtherportion of path 174, also oblique to the downstream direction, back tothe receiving transducer, in this case the second or downstreamtransducer 144. The receiving transducer converts the ultrasonic wavesto electrical signal. Because the path from the driven transducer to thereceiving transducer has a component parallel to the direction of flowof the blood, the time of flight of the ultrasonic waves is influencedby the velocity of the blood according to the well-known Doppler effect.This causes the phase of the received ultrasonic waves to vary with theblood velocity, and thus with the flow rate. Because the housingelements 112 and 113 are rigid, the geometry of the system is fixed. Asused in this disclosure, the term “rigid” should be understood asmeaning that the housing elements do not distort in normal operation ofthe pump to a degree which would appreciably affect the phase differencebetween the received and emitted ultrasonic waves. The mathematicalrelationships used to convert phase difference to flow velocity, and toconvert flow velocity to flow rate, are well known. The circuits used tomeasure phase difference are also well known and accordingly are notfurther described herein.

Because the flow measurement is performed by the ultrasonic sensorsmounted in the pump housing, there is no need for a separate flowmeasurement device mounted along the outflow cannula. Moreover, becausethe connection between the flow sensor and the control unit is madethrough conductors of the same driveline used to convey power to thepump, there is no need to implant a separate cable leading to a flowsensor.

In a variant of the embodiment discussed above, the connectors 197 and127 may be integrated into a single connector, mated to a singledriveline interface on the pump housing. In yet another variant,conductors of the driveline which convey power pump coil system may alsobe used to convey ultrasonic frequency electrical signals to and fromthe PCB or the transducers in a multiplexing arrangement. In yet anotherembodiment, the PCB 150 may also be used to convey power to theelectrically driven elements of the pump itself, such as the coil set109 schematically shown in FIG. 2.

Also, although the pump depicted in FIGS. 1-7 is a radial-flow impellerpump, the invention can be applied in other pumps, such as an axial-flowimpeller pump as depicted in the aforementioned U.S. Pat. No. 8,419,609,and in conjunction with pumps such as diaphragm pumps and piston pumps.Further, although the invention has been described with reference to anultrasonic flow sensor having two ultrasonic transducers, other types offlow sensors may be mounted to the pump housing. For example, flowsensors which measure the rate of heat transfer to the flowing bloodfrom a heated element can be employed.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

The invention claimed is:
 1. A blood pump comprising: an implantablepump for pumping blood having a rigid housing, a flow path extendingwithin the housing in upstream and downstream directions and at leastone movable element within the housing for impelling blood along theflow path; a first housing element and a second housing element withinthe housing having respectively a first interior surface and a secondinterior surface, the first and second interior surfaces confrontingeach other and cooperatively defining therebetween at least a portion ofthe flow path, the first housing element having an exterior surfacedefining a cavity external to the flow path; wherein the first housingelement defines a first platform surface facing downstream at an obliqueangle to a flow path axis and a second platform surface facing upstreamat an oblique angle to the flow path axis, in which the first and secondplatform surfaces overlie the portion of the flow path, in which thefirst and second platform surfaces are within the cavity and extendrespectively from first and second portions of the first interiorsurface which face in a same direction and confront the second interiorsurface, and wherein a first flow sensor is mounted to the firstplatform surface at an oblique angle to the flow path axis and a secondflow sensor is mounted to the second platform surface at an obliqueangle to the flow path axis.
 2. The blood pump of claim 1, wherein thesensor includes at least two ultrasonic transducers.
 3. The blood pumpof claim 1, wherein each of the platforms has a slope of substantially45 degrees to the flow path axis.
 4. The blood pump of claim 2, whereinthe ultrasonic transducers are mounted to the platforms with anadhesive, at a slope of substantially 45 degrees to the flow path axis.5. The blood pump of claim 2, the first and second transducers beingarranged so that an ultrasound signal emitted from the first transducerpasses through the flow path to the second housing element, reflectsfrom the second housing element and is received by the secondtransducer, wherein the ultrasound signal is emitted and received at anangle oblique to the flow path axis.
 6. The blood pump of claim 5,wherein the pump is a rotary pump.
 7. The blood pump of claim 1 whereinthe housing further includes a cover overlying the cavity in the firsthousing element.
 8. The blood pump of claim 1 further comprising anelectronic circuit disposed within the cavity and connected to thesensors.
 9. The blood pump of claim 1, wherein the sensors are mountedadjacent an inflow end of the flow path.
 10. The blood pump of claim 1,wherein the sensors are mounted adjacent an outflow end of the portionof the flow path.
 11. The blood pump of claim 1 wherein the pumpincludes one or more electrical elements for moving the movable element,the blood pump further comprising an external control unit operative topower the electrical elements, and a driveline for connecting the pumpand at least one ultrasonic sensor to the external control unit.
 12. Theblood pump of claim 11, wherein the sensors are connected to theexternal control unit through the driveline.
 13. The blood pump of claim12, wherein the driveline is the only connection between the pump andthe external control unit.